The long term goal of our research is twofold 1) to understand how basal forebrain cholinergic neurons (BFC) together with other local non-cholinergic neurons participate in specific basal forebrain circuits and 2) to understand the fundamental mechanisms of transneuronal interactions between BFC neurons and their afferents. To achieve this goal, we will continue the anatomical and chemical characterization of afferents to specific subpopulations of BFC neurons. Specific attempts will be made to assess whether projections from different brainstem monoaminergic nuclei, the midbrain reticular formation and from the prefrontal cortex terminate on cholinergic forebrain neurons. As a logical extension of these experiments we will identify the sources of afferents to local GABAergic and peptidergic neurons which are connected with the cholinergic neurons. These studies will be aided by using in vivo and in vitro (anterograde and retrograde) tracer techniques (PHA-L, Lucifer Yellow, etc), and double immunolabeling methods in various combinations at both the light and electron microscopic level. We will also characterize how the disruption of monoaminergic afferent systems affects neurotransmitter regulation in the BFC and other basal forebrain neuronal populations. Specifically, we will study how gene expression for monoaminergic and NGF receptors, neurotransmitter enzymes (GAD, ChAT) and neuropeptides (NPY, somatostatin) is regulated through removal of monoaminergic afferents. Furthermore. we will test the hypothesis whether the decrease of ChAT activity after monoaminergic depletion Is mediated through the forebrain GABAergic system by monitoring GABAergic activity. Defining the precise Input-output relations of the BFC will provide the necessary database for pharmacological, behavioral or clinical studies designed to understand the specific role of BFC in cortical activation or reward related events, functions in which the BFC has been implicated. Moreover, our study may provide additional clues to the mechanism of monoaminergic/cholinergic inerteractions in the basal forebrain. Such interactions may also be important in learning and memory processes, and their loss may contribute to the cognitive decline seen in aging and in dementing diseases.